Many devices used in biomedical applications require that the bulk of the device have one property, while the surface of the device has another property. For example, contact lenses may have high oxygen permeability through the lens to maintain good corneal health. However, materials that exhibit exceptionally high oxygen permeability (e.g. polysiloxanes) are typically hydrophobic and will adhere to the eye. Thus, a contact lens generally has a core or bulk material that is highly oxygen permeable and hydrophobic, and a surface that has been treated or coated to increase hydrophilic properties, thereby allowing the lens to freely move on the eye without adhering excessive amounts of tear lipid and protein.
In order to modify the hydrophilic nature of a relatively hydrophobic contact lens material, a contact lens can be treated with a plasma treatment. For example, a high quality plasma treatment technique is disclosed in PCT Publication No. WO 96/31793 to Nicholson et al. Some plasma treatment processes, however, require a significant monetary investment in certain equipment. Moreover, plasma treatment requires that the lens be dry before exposure to the plasma. Thus, lenses that are wet from prior hydration or extraction processes must be dried, thereby imposing added costs of obtaining drying equipment, as well as added time in the overall lens production process. As a result, a number of methods of consistently and permanently altering the surface properties of polymeric biomaterials, such as contact lenses, have been developed. Some of these techniques include Langmuir-Blodgett deposition, controlled spin casting, chemisorptions, and vapor deposition. Useful examples of Langmuir-Blodgett layer systems are disclosed in U.S. Pat. Nos. 4,941,997; 4,973,429; and 5,068,318.
A more recent technique used for coating electronic devices is a layer-by-layer (“LbL”) polymer absorption process, which is described in “Investigation of New Self-Assembled Multilayer Thin Films Based on Alternately Adsorbed Layers of Polyelectrolytes and Functional Dye Molecules” by Dongsik Yoo, et al. (1996). The process described in this article involves alternatively dipping hydrophilic glass substrates in a polyelectrolyte solution (e.g., polycations such as polyallylamine or polyethyleneimine) and then in an oppositely charged dye solution to form electrically conducting thin films and light-emitting diodides (LEDs). After each dipping, the substrates are rinsed with acidic aqueous solutions. Both the dipping and rinsing solutions have a pH of 2.5 to 7. Prior to dipping, the surfaces of the glass substrates are treated in order to create a surface having an affinity for the polyelectrolyte.
Similar to the above process, two other processes are described by “Molecular-Level Processing of Conjugated Polymers” by Fou & Rubner and Ferreira & Rubner, respectively. These processes involve treating glass substrates that have hydrophilic, hydrophobic, negatively, or positively charged surfaces. The glass surfaces are treated for extended periods in hot acid baths and peroxide/ammonia baths to produce a hydrophilic surface. Hydrophobic surfaces are produced by gas-phase treatment in the presence of 1,1,1,3,3,3-hexamethyldisilazane for 36 hours. Charged surfaces are prepared by covalently anchoring charges onto the surface of the hydrophilic slides. For example, positively charged surfaces are made by further treating the hydrophilic surfaces in methanol, methanol/toluene, and pure toluene rinses, followed, by immersion in (N-2 aminoethyl-3-aminopropyl)trimethyloxysilane solution for 12 to 15 hours. This procedure produces glass slides with amine functionalities, which are positively charged at a low pH.
In addition to the above-described techniques, U.S. Pat. Nos. 5,518,767 and 5,536,573 to Rubner et al. describe methods of producing bilayers of p-type doped electrically conductive polycationic polymers and polyanions or water-soluble, non-ionic polymers on glass substrates. These patents describe extensive chemical pre-treatments of glass substrates that are similar to those described in the aforementioned articles.
The methods described above generally relate to layer-by-layer polyelectrolyte deposition. However, these methods require a complex and time-consuming pretreatment of the substrate to produce a surface having a highly charged, hydrophilic, or hydrophobic nature in order to bind the polycationic or polyanionic material to the glass substrate.
To reduce the complexity, costs, and time expended in the above-described processes, a layer-by-layer polyelectrolyte deposition technique was developed that could be effectively utilized to alter the surfaces of various materials, such as contact lenses. This technique is described in co-pending U.S. Patent Application entitled “Apparatus, Methods, and Compositions for Modifying Surface Characteristics”. In particular, a layer-by-layer technique is described that involves consecutively dipping a substrate into oppositely charged polyionic materials until a coating of a desired thickness is formed. Nevertheless, although this technique provides an effective polyelectrolyte deposition technique for biomaterials, such as contact lenses, a need for further improvement still remains. For example, with this layer-by-layer dipping process, a coating could require multiple dipping steps that take a substantial amount of time to apply. As a result, manufacturing costs can often be increased due to the amount of time and dipping required to sufficiently coat the substrate.
As such, a need currently exists for an improved method of coating a material, such as a contact lens, with polyelectrolyte (polyionic) layers. In particular, a need exists for an improved polyionic deposition technique that requires less time and dipping than the previously-described layer-by-layer deposition technique.